For example, a frame member for a seat for transportation equipment such as an automobile, an aircraft and the like is strongly required to be reduced in weight in view of fuel consumption improvement, carbon dioxide emission control or the like, and thus, high strength of a steel material used for forming a frame member for a seat is in demand. On the other hand, the frame member for a seat is also required to have not only high strength but also high toughness (also including ductility) in view of impact absorbing properties owing to deformation or the like. As techniques satisfying these demands, for example, high-strength steel plates disclosed in Patent Literatures 1 to 3 are known.
Each of the high-strength steel plates disclosed in these Literatures assumes control on an addition amount of an alloy element other than carbon, and it is made to contain, for example, Mn, Mo, Cr, or the like in a predetermined amount or more to secure a predetermined hardness or ductility. Then, for use as a steel material for an automobile or the like, the high-strength steel plate is finally cold-rolled down to a thickness of 1.2 mm, but heat treatment performed at a step before the cold rolling is to hot-roll a steel slab to a thickness of 3.2 mm. That is, since the techniques disclosed in these Literatures are techniques for obtaining a steel plate with a thickness of several mm or thicker, it is necessary to achieve evenness of a microstructure including a plate-thickness direction in the steel plate in the heat treatment, and thus, control on an addition amount of an alloy element is an important factor.
On the other hand, in Patent Literatures 4 to 5, techniques of achieving high strength of ordinary low-carbon steel have been disclosed. Patent Literature 4 discloses a technique proposed in order to solve such a problem that, since tempering property of ordinary low-carbon steel was poor in the previous technique, when a martensite was utilized as an originating structure, an uneven duplex grain structure was produced during an annealing time so that a predetermined high-strength and high-ductility steel material could not be obtained. Therefore, in Patent Literature 4, after ordinary low-carbon steel is tempered to achieve martensite phase of 90% or more, an ultra-fine crystal grain ferrite structure with grain diameters of 1.0 μm or less is obtained by performing cold-rolling with a total reduction ranging from 20% to less than 80% and performing annealing. Patent Literature 5 is the technique which has been proposed by the present applicant, where high strength is achieved by performing working process for elevating internal stress, such as press forming, and achieving refinement and duplex grain sizing of a metal structure of low-carbon steel by heat treatment.